When a key part of a building is entombed by falling sand, it loses its decor value.
I first noticed this happening in a regular Survival game. I tested it further in sandbox mode and found that this bug occurs with mess tables, small Sculpting Blocks (multi-block), Oxygen Diffusers (negative decor value), and Rock Granulators (large, multi-block). The bug seems to affect the duplicants' average decor value for the current cycle as well. To clarify, the value for Current Environmental Decor in a Duplicant's status tab would be consistent with the decor overlay; I'm not completely sure whether it really affects the value of average decor per cycle but I assume it would.
However, something interesting happens with the Rock Granulator. Even when partially entombed, it still provides a decor value under a certain condition. If the tile that requires a connection to a wire (second tile of the bottom row) is entombed, the entire building loses its decor value. The other tiles do not matter; if they are entombed, the building will still provide a negative decor value during and after entombment.
A Liquid Pump loses its decor value if its bottom left corner is entombed. This is underneath, but not coincident with, its power port. It is also not coincident with its liquid output. It will not lose its decor value if its bottom right corner is entombed. A Shower's key block is also its bottom left block, and if it is entombed it will lose its decor value, even after being excavated.
The bug does not seem to occur for debris or plants (e.g. Bluff Briars).
A workaround for the bug is to reload the save file. The decor value of the building will be fixed. Closing and reopening the decor overlay does not fix the bug.
EDIT: This bug may be related to another bug I posted stating that airlocks stop providing a decor penalty when closed.
Steps to Reproduce
Dig the blocks under a Sand tile to cause it to entomb a building with a decor value. Remove the sand entombing the building, and you may notice that the building does not have a decor contribution in the decor overlay. The block that must be entombed varies from building to building. Easiest to reproduce using a Mess Table. Works in sandbox mode.
This issue has not been confirmed by a developer yet.